The invention is directed to a process for producing a line of weakness in a cover element through removal of material by laser as is known generically from patent publication DE 10 2013 104 138 B3.
In the present day, the use of airbag systems in vehicles or other means of transport is generally standard. Airbags are arranged as inconspicuously as possible behind parts of the interior trim of the vehicles so as not to infringe upon the aesthetic sensibility of passengers. The interior trim, hereinafter “cover element”, generally comprises stable, two-dimensionally extensive molded parts of plastic or composite materials. Since the airbags are ejected through the cover element in the event of activation, airbag flaps must be provided in the cover element. The airbag flaps are often formed by specially constructed areas of the cover element having predetermined breaking points which are introduced along the edges of the airbag flaps and which ensure that the cover element tears open reliably and in a defined manner.
In high-quality constructions of cover elements, a stable, shape-imparting layer is often provided with covering materials through which the surface of the cover element presenting toward the passenger space is visually and tactilely experienced. These covering materials are generally foam and flexible, thin-walled materials such as plastic sheeting, imitation leather, textile knits, microfiber webs or genuine leather. For reliable deployment of the airbag, the covering materials must also be provided with predetermined breaking points in the area of the airbag flaps. For this purpose, lines of weakness are introduced in precisely the same manner as in the stable, shape-imparting layers. For visual reasons, these lines of weakness are generally introduced from the back side of the covering material that is not visible from the passenger compartment. In addition to an exactly definable residual tear strength of the line of weakness, very high quality standards for the surfaces are only met when the line of weakness is not discernible visually or tactilely on the visible side of the covering material presented to the passenger.
There are a variety of methods for introducing the lines of weakness.
DE 10 2006 054 592 B3 discloses a method in which weakened areas are introduced by laser in a cover element (referred to in the above-cited publication as decorative composite) comprising layers. A decorative composite generally includes a decorative material on the visible side and a decorative material support, between which are arranged one or more layers of a padding. The weakening is introduced in a plurality of successive work cycles. In a first work cycle, a non-penetrating pre-weakening of the decorative support is carried out and a post-weakening in the form of perforation holes penetrating the decorative support is carried out in the pre-weakened areas in at least one second work cycle. There remain between the pre-weakened areas or perforation holes, unweakened bridges which are post-weakened in a second work step with at least one pocket hole. The above-cited publication does not provide specifics on the execution of the perforation holes or on adapting the perforation depth to variations in thickness of the decorative materials.
A further laser process is described in the publication DE 11 2006 000 443 T5. In this case, perforation holes are introduced into cover elements (referred to as airbag covers in the above-cited publication), e.g., instrument panels, by means of a pulsed laser beam. An instrument panel is formed of a base layer and a thinner skin layer (visible side) of plastic. The perforation holes are introduced from the base layer side and can extend into the skin layer. Deep main recesses and shallower auxiliary recesses are formed by laser pulses with different pulse durations. This publication also does not mention adapting the perforation depth to possible thickness variations in the layers.
A process in which a line of weakness is produced by perforating a cover element (referred to as trim part) with a leather decorative layer by means of a pulsed laser is disclosed in Laid Open Application WO 2005/049261 A1. The perforation is formed by a plurality of individual perforation holes which are arranged along the line of weakness so as to be separated by remaining bridges.
As in the publications acknowledged as prior art in the above-mentioned WO 2005/049261 A1, the line of weakness is introduced during a movement of the laser relative to the covering material that is executed once, and perforation hole holes are made one at a time successively during this one-time relative movement. The depth of the perforation is influenced and the remaining residual wall thickness of the covering material is adjusted, respectively, by correspondingly adapting the pulse duration and the laser power in relation to the speed of the relative movement.
Further, steps are suggested for minimizing the heat load on the covering material during laser machining. To this end, the perforation holes arranged consecutively on the line of weakness are produced by short and ultrashort laser pulses, respectively, with corresponding intervals between the individual laser pulses. In accordance with the disclosed process, it must be assumed that these intervals are achieved by reducing the pulse frequency so that the energy inputs of the laser pulses which otherwise impinge at higher frequency cannot add up over time.
As opposed to the processes mentioned above, a line of weakness with defined tear strength and appreciably reduced range of variation of the tear strength can be produced by this process. The machining process is protracted owing to the intervals between the laser pulses and the reduced pulse frequency of the laser for this purpose.
A similar process is disclosed in patent publication DE 10 2007 013 108 B3. The cover element to be perforated in this case has alternating areas of thicker and thinner material over the course of the perforation line to be produced in which the pulsed laser is operated in a first laser regime or second laser regime, respectively, to generate the desired residual wall thicknesses of the pocket holes. The areas of material thickness are detected in the first laser regime by means of a defined, uniform indication pulse sequence of the laser beam which is applied at the start of every perforation hole. In the area of less thick material, the indication pulse sequence is sufficient to produce a pocket hole until transmitting laser radiation is detected. A sufficient perforation cannot be achieved by the indication pulse sequence in the thicker material area, so that the second laser regime with a higher laser power is switched to at the end of the indication pulse sequence for producing the pocket hole. This process is inherently unsuited to adjust the thickness of the residual wall.
The above-cited patent publication DE 10 2013 104 138 B3 describes a process for introducing a defined line of weakness in a covering material by removal of material in which the line-shaped guiding of a pulsed laser beam such as is known from the above-cited publications is a scanning motion which is repeated a plurality of times and in which only one laser pulse is emitted for each impingement location along the line. The parameters of the laser pulse are selected in such a way that this laser pulse causes an input of energy which leads to a heating of the covering material to a temperature above an ablation threshold at the respective impingement location, but the temperature in areas of the covering material adjoining the respective impingement location is kept below a limiting temperature that would lead to changes in the structure of the covering material.
The multiple repetition of the scanning movement can be carried out until a small residual wall thickness is achieved so that the laser radiation that is now transmitted is detected by a sensor. When the minimum permissible residual wall thickness is reached at an individual impingement location, a spatially resolved cutoff of the laser beam takes place during the scanning movement.
The speed of the scanning movement and the pulse repetition frequency of the pulsed laser beam are adapted to one another so that only one laser pulse impinges per impingement location.
Alternatively, the laser beam can advantageously be switched on and switched off during the repeated scanning movement corresponding to a fixed regime. The line of weakness introduced along the line has the form of a slit/bridge line with an alternating succession of slits and bridges.
By detecting when a minimum residual wall thickness is reached per impingement location and effecting a corresponding spatially resolved cutoff, a line of weakness having a tear strength that is constant along the length thereof regardless of fluctuations in material thickness can be produced. Consequently, the amount of tear resistance cannot be influenced via the remaining residual wall thickness in this case and must consequently be adjusted via the quantity of impingement locations or the perforation holes which are formed so as to penetrate up to a residual wall and by the spacing and bridge lengths of these perforation holes. Perforation holes or slits with differing residual wall thicknesses or with residual wall thicknesses which are not yet detectable because a possibly transmitting laser radiation is still below the threshold of the sensor cannot be produced with this process. Accordingly, the residual wall thickness can only be used to a very limited extent for adjusting a desired tear strength or degree of weakness, and the process is wholly unsuitable for many materials because a minimum residual wall thickness along the entire line of weakness does not guarantee that the latter will be consistently invisible.
In a process and an airbag cover disclosed in patent publication EP 0 991 551 B1, a line of weakness is generated by a succession of a plurality of consecutively produced perforation holes by means of a pulsed laser beam. The perforation holes can be introduced so as to overlap one another or so as to be spaced apart from one another by remaining bridges. The perforation holes are completed as soon as a transmission of laser radiation is detected through the remaining bridge. Lines of weakness, sections in the line of weakness or individual holes can also be produced in addition to the perforation without completely perforating the material. The depth of this non-perforating weakening can be estimated and adjusted based on the quantity of pulses and/or the pulse duration required for producing a perforation. The publication does not disclose more precise instructions for adjusting the depth.